Summary: The goal of this project is to understand the molecular basis of lymphocyte differentiation from na?ve to memory, and immunological memory. As the first step toward this goal, we have analyzed gene expression profiles of human naive and memory CD4+ T cells, and identified genes that are differentially expressed in one population over the other. Human CD4+ naive and memory T cells are phenotypically well defined subsets, i.e. naive CD4+ T cells expressing CD45RA and memory CD4+ T cells expressing CD45R0. We used immunomagnetic separation isolating large number of highly pure naive and memory cells from peripheral blood. Messenger RNA was isolated from naive and memory T cells and used for gene expression analysis. Gene expression profile and differentially expressed genes were analyzed by cDNA microarray. CD4+ na?ve and memory T cells express 2005 (11%) and 1792 (10%) of genes over 18,000 human genes analyzed, respectively. A total of 23 genes (1.3% of expressed genes) were over-expressed in naive cells, and 151 genes (8.3% of expressed genes) were over-expressed in memory T cells. Most of these genes are new genes, and some of them express Alu or other repeated sequences at the 3' end which appears to be more frequent in memory cells. Currently, we are characterizing these differentially expressed genes by Northern, in situ hybridization, and sequencing. We will select a few of genes for detailed analysis, such as making antiserum or monoclonal antibody to the protein, and assessing functions of the proteins in lymphocyte in vitro and in animal with the homologous genes in vivo.